When Isaac Newton saw the apple fall he understood
how gravity works and he was able to formulate a law for it. He
applied this
law to explain the orbit of the moon travelling around the earth.
At the same time he could explain how the force of gravity between
two bodies diminishes
when the distance between them increases. Later, it was discovered
that there is a similar decrease of the electric force between
two bodies carrying
electric charges as they are separated from each other. But what
is a force? How do forces arise? How can they act in empty space – in
a vacuum? Almost exactly a hundred years ago, Albert
Einstein realized that light
can be described as a bunch of particles, called photons, which
move from one electrically charged body to another and mediate
the electromagnetic
force between the bodies. They mediate a force in the same way
as the football that the goalkeeper cannot stop or as the cannon
ball that can destroy a
tower of a fortress. We can also remind ourselves of how Baron
von Münchhausen
threw himself and grabbed a cannon ball to travel over a wall.

Why does the force diminish with increasing
distance? The cannon ball meets resistance from the air but the
force due to the motion of photons also decreases in vacuum. This
is because according to quantum theory a beam of photon particles
can also be thought of as a wave (as in the standard description
of light). The further away from the source you are, the smaller
the section of the wave that will hit you. When the wave that appeared
when Krakatoa exploded in 1883 hit the coastline
of Sumatra it caused enormous damage, but when it hit the coast
of Africa much further away it was just a swell in the water. We
can also understand the force of gravity in the same way as the
electromagnetic force. We have not been able to discover the force
particles here, but we are convinced that they exist.

At the same time as the physicist wants to
understand the fundamental forces that act in Nature, he or she
wants to understand what the fundamental building blocks are. We
have divided up matter into atoms, which have been further subdivided
into electrons and the nucleus, which is made of protons and neutrons.
It became clear very early on that within the nuclei there must
be different interactions, the weak nuclear force responsible for
the radioactive decays, and the strong nuclear force holding the
nucleus together even when the repulsive electric force between
the protons tries to separate them. These forces act over very
short ranges, ranges as improbably small as the nucleus itself.
To understand these forces and to understand the fundamental building
blocks of Nature have been the great task of particle physics for
the last fifty years. This year's Nobel Prize completes the
picture that the work behind several earlier prizes initiated and
as a result we now know the fundamental building blocks and we
have a description of the four fundamental forces.

One of these earlier discoveries was the understanding
that protons and neutrons are composite objects, which are made
of even more fundamental particles called quarks. It was found
that quarks come in various species that have different types of
charges, which came to be called colour charges. These charges
are in some sense analogous to electric charges, so it was natural
to believe that the quarks should behave like the electrons. However,
unlike electrons, no free quarks were ever discovered. Perversely,
it seemed as if the force increased as quarks were separated. Conversely,
plenty of evidence accrued that when two quarks got close to each
other they could hardly feel each other. This behaviour is called "asymptotic
freedom." Could a theory of the quarks behave like that?
This behaviour of the force between quarks seemed to be outside
the realm of any theory of the kind that had so successfully explained
the electromagnetic force. So around 1970 particle physics stood
in front of a great dilemma. Common sense, and all calculations
that were attempted told us that the force between quarks should
behave in a manner that contradicted the experimental facts. In
the end the issue came down to one specific question. Does any
theory predict a minus sign in the right place? All theories that
were tested gave the incorrect positive sign.

In 1973, David
Gross and Frank Wilczek and
David Politzer considered
a novel class of theories. To the surprise of the world and to
their own great astonishment they found the
result –11/3 that signalled that these theories are asymptotically
free. Seldom has a negative result had such a positive effect!
A theory for the strong force between the quarks could now quickly
be formulated and a detailed comparison with experiments could
be performed. During the last fifteen years experiments at large
accelerators have confirmed the theory with great accuracy. The
theory of Gross, Politzer and Wilczek successfully describes the
physics of quarks, the matter from which we are to a very large
extent built. Since the discovery, further research has shown that
these theories are unique. No other theories can account for the
experimental picture and it is wonderful to know that Nature has
chosen the only theory that we have found to be possible.

Professor Gross, Professor Politzer, Professor
Wilczek.

You have been awarded the 2004 Nobel Prize in Physics for your
discovery of asymptotic freedom in the theory of the strong interactions.
It is an honour for me to convey the warmest congratulations of
the Royal Swedish Academy of Sciences. I now ask you to step forward
to receive your Nobel Prizes from the hands of His Majesty the
King.